There exists a class of devices exemplified by U.S. Pat. No. 4,020,830 to Johnson et al., dated May 3, 1977, entitled SELECTIVE CHEMICAL SENSITIVE FET TRANSDUCERS. These devices feature electrical conduction characteristics which are modulated by the interaction of a chemically selective system with ambient materials. In accordance with the Johnson patent, a substrate layer carries respective drain and source regions, separated by a region or channel over which is disposd an insulating layer and a chemically selective system for specified interaction with predetermined ambient materials. The chemically selective system generally takes the form of a membrane which interacts with materials, and modulates the drain to source electrical current based on concentrations of the specified ambient substances.
The Johnson et al. patent contemplates chemically selective systems for measuring various types of ambient conditions, including gas concentrations, ion activity, immunochemical concentrations, concentrations of enzymes, and the like, and indeed many such applications have engendered widespread interest in a variety of disparate fields. While the nomenclature in the art has tended to designate these respective applications separately, for example using the designation "CHEMFET" for chemically selective membrane devices, "ISFET" for ionically reactive devices, 37 IMMUNOFET" for immunologically reactive devices, and so forth, for purposes of this application the term "chemfet", or simply "device" shall be utilized generically to encompass all such apparatus, irrespective of the type of sensing or reaction utilized, the character of the membrane employed, or the nature of the ambient substance to be monitored. Likewise, the term "chemfet" or "device" as used herein shall embrace transistor type, diode type, or the like other devices which feature similar conductivity modulation based on membrane-substance interactions.
In recent times, much effort has been expended in the development of device configurations in manufacturing processes which will facilitate large-scale production of reliable, stable, and well-calibrated devices. For example, device encapsulation, membrane formulation, and membrane disposition have proven to be formidable technical problems.
It is a general object of the principles of the present invention to provide encapsulation apparatus and compatible assembly processes for the production of chemfet devices which have high reliability, well quantified specifications, improved physical integrity, and a reasonable operational lifespan.
The packaging or encapsulation problems attendant to chemfet devices are highly demanding and somewhat unique in the semiconductor fabrication field. The devices themselves are quite small, and rely in their operation on the exposure of even smaller chemically sensitive gate portions with ambient solution environments. With the exception of the miniature gate portions, however, the remainder of the semiconductor chip and associated connections, including conductive lines, chip edges, and bonding pad connections, must be hermetically sealed. Failure so to seal these portions of the device engenders corrosion problems and eventual device degradation and failure, and, in the worse case, extraneous electrical paths between conductors in the device system. Furthermore, with particular reference to biomedical applications, exposure of any portion of the system, other than the chemically sensitive gate region, raises the risk of untoward contamination of the biological materials being monitored.
It is accordingly an object of the present invention to provide encapsulation systems and techniques which are compatible with the dimensions and fragility of the devices and especially their chemically sensitive membrane systems.
A popular, although tedious technique for chemfet device encapsulation involves utilization of a hollow, catheter style flexible tube. Small gauge wires are threaded through the catheter, and at one end are soldered to any of a variety of commercially available connectors for plug in to monitoring circuitry. At the other end, the wires are epoxied and cut, leaving bare strands for connection of the chip. Next, the semiconductor chip carrying one or more devices is epoxied to the catheter and small (e.g., 0.001 inch diameter) wires are ultrasonically bonded between the chip bonding pads and the exposd wire end strands from the catheter. Once correct electrical contact has been verified, a thixotropic epoxy is applied by hand to cover all of the chip and wires, but leaving exposed small open areas around the chemically sensitive gate membrane portion of the devices.
The foregoing procedure has for the most part been moderatly successful in the production of working devices, but clearly involves considerable time, training, and skill on the part of the assembler. Additionally, the essentially "hand-made" nature of each device yields a nonuniform product; this nonuniformity often leads to variation in performance as various membranes are applied to the units. Furthermore, the ultrasonic wire bonding step necessarily produces a nonplanar loop, which also must be encapsulated, but the encapsulation of which engenders further structural risk to the device during the encapsulation process. Finally, the nonplanar nature of this encapsulation further contributes to the functional nonuniformity of the devices so produced.
It is an important object of the principles of the present invention to obviate the difficulties, drawbacks, and uncertainties essentially attendant to the foregoing "hand-made" chemfet encapsulation techniques, as well as the functional uncertainties and nonuniformities necessarily resulting therefrom.
A technique which has recently found favor in the semiconductor industry for the enclosure of semiconductor chips is the so-called tape automated bonding process. See, for example, an article in Solid State Technology, March 1978, by Cain entitled "Beam Tape Carriers--A Design Guide", or an article in EDN, Aug. 20, 1977 by Palstone entitled "Beam--Tape Technology". Generally, conventional tape automated bonding techniques employ the use of a flat copper lead pattern, often defined on a substrate by photolithographic process, to which the semiconductor chip is bonded. Thereupon, either the chip and beam leads are separated and used, or the entire assembly is suitably fully enclosed.
It is an object of the present invention to adapt tape automated bonding techniques to the encapsulation of chemfet-style devices.